Generally, a hydraulic system of construction machinery includes an engine which generates power, a main hydraulic pump which is driven by transmitted power of the engine to eject hydraulic oil, a plurality of actuators which performs an operation, a manipulating unit which is manipulated to operate an actuator of a desired operating machine, and a main control valve which distributes hydraulic oil required by the manipulation of the manipulating unit to a corresponding actuator.
In the manipulating unit, a required command is formed in accordance with a manipulating displacement manipulated by an operator and a flow of the hydraulic oil ejected from the hydraulic pump is controlled by the required command. Examples of the manipulating unit include a joy stick and a pedal.
Further, a rotation torque of the pump needs to vary in order to eject the hydraulic oil from the main hydraulic pump. The torque is referred to as a pump torque. The pump torque T is calculated by a product of a volume of the pump and a pressure P formed in the hydraulic oil. The above-mentioned volume of the pump is a flow of the hydraulic oil which is ejected for one rotation of a shaft of the pump.
According to the above-described hydraulic system which is known in the related art, the hydraulic pump distributes the hydraulic oil ejected from one or two main pumps to each actuator in accordance with control of a main control valve. That is, a pressure of the hydraulic oil ejected from the main control valve necessarily goes through a pressure loss while the hydraulic oil passes through the main control valve and various valves. As a result, energy efficiency may be lowered.
In the meantime, a hydraulic system is illustrated in FIG. 1 of the following Patent Document. More specifically, the hydraulic system disclosed in Patent Document includes a plurality of actuators and a plurality of pumps. Further, each pump is exclusively assigned to each actuator. Furthermore, each control valve is provided on a hydraulic line of each actuator. Each control valve is controlled to determine a flow of the hydraulic oil provided to each actuator and a flowing direction of the hydraulic oil.
However, in the above-described hydraulic system disclosed in the Patent Document, when the control valve is adjusted to operate the corresponding actuator, a pressure loss of the hydraulic oil is generated. The pressure loss adversely affects the fuel efficiency of an excavator.
Further, among the plurality of actuators, a specific actuator may be in an idle state in accordance with the state of an operation of the excavator. However, in spite of the idle state, the pump is continuously driven which causes energy to be consumed.